The present invention relates to a switching power supply circuit and, more particularly, to an auxiliary power supply circuit for supplying starting power to a control circuit for ON/OFF-controlling a switching transistor.
FIG. 2 shows a conventional switching power supply circuit for converting an AC voltage into a DC voltage.
Reference numeral 1 denotes a rectifier for rectifying an AC input voltage. Reference symbol Q1 denotes a switching transistor for converting a DC voltage output from the rectifier 1 into an AC voltage having a high frequency, e.g., several 10 kHz; and T1, a transformer having a primary winding L1 on the input side, and a secondary winding L2 on the output side. The primary winding L1 is insulated from the secondary winding L2. The transformer T1 induces a predetermined AC voltage in the secondary winding L2 by using a high-frequency AC current flowing in the primary winding L1. Reference numeral 2 denotes a rectifying/smoothing circuit for rectifying an AC voltage induced in the secondary winding L2 of the transformer T1 and applying the resultant DC output voltage to a load (not shown). The rectifier 1, the transistor Q1, the transformer T1, and the rectifying/smoothing circuit 2 constitute an AC/DC converter.
Reference numeral 3 denotes a PWM (Pulse Width Modulation) control circuit for outputting a control signal for ON/OFF-controlling the transistor Q1. The PWM control circuit 3 compares a DC output voltage output from the rectifying/smoothing circuit 2 with a triangular voltage generated by an oscillator (not shown) incorporated in the circuit 3, and changing the pulse width of a pulse signal constituting a control signal to change the ON/OFF time of the transistor Q1, thereby keeping the DC output voltage constant. Reference numeral 4 denotes a starting auxiliary power supply circuit constituted by a resistor R1. The resistor R1 serves to supply power to the PWM control circuit 3 when an AC input voltage is applied, i.e., when application of a DC voltage from the rectifier 1 is started. Reference symbol L3 denotes an auxiliary winding L3 on the primary side of the transformer T1; and D1, a diode for rectifying an AC voltage induced in the auxiliary winding L3. Reference numeral 5 denotes a voltage regulator for regulating a DC voltage rectified by the diode D1 to a predetermined voltage. Reference symbol D2 denotes a diode for preventing a reverse current to the voltage regulator 5. These components constitute an auxiliary power supply circuit 6 for a normal operation. The auxiliary power supply circuit 6 supplies power to the PWM control circuit 3 after the transistor Q1 starts a switching operation.
An operation of the above-described conventional switching power supply circuit will be described next.
When an AC input voltage is applied to the switching power supply circuit, a DC voltage is applied from the rectifier 1. At this time, since the transistor Q1 is OFF and does not start switching, no voltage is generated in the auxiliary winding L3 of the transformer T1. For this reason, when an AC input voltage is applied to the switching power supply circuit, i.e., when application of an DC voltage from the rectifier 1 is started, the starting auxiliary power supply circuit 4 constituted by the resistor R1 supplies a starting current to the PWM control circuit 3 by using the output from the rectifier 1. With this operation, the PWM control circuit 3 is started.
Immediately after the starting operation, the PWM control circuit 3 outputs a control pulse signal having a long ON time because the DC output voltage from the rectifying/smoothing circuit 2 is lower than the triangular voltage. The transistor Q1 starts a switching operation in accordance with this control signal. With this switching operation of the transistor Q1, the DC voltage from the rectifier 1 is converted into a high-frequency AC voltage. After the AC voltage is increased or decreased to a predetermined voltage by the transformer T1, a stable DC output voltage is output from the rectifying/smoothing circuit 2. As the DC output voltage from the rectifying/smoothing circuit 2 is increased by the initial switching operation with a long ON time, the PWM control circuit 3 outputs a control pulse whose ON Time is shortened in reverse proportion to the increase in DC output voltage. Therefore, the DC output voltage from the rectifying/smoothing circuit 2 is stabilized. By changing the ON/OFF time of the transistor Q1 in this manner, the DC output voltage is stabilized to become a constant voltage.
When an AC current starts to flow in the transformer T1 after the transistor Q1 starts the switching operation, an AC voltage is induced in the auxiliary winding L3 of the transformer T1. A constant voltage is generated by inputting this AC voltage to the voltage regulator 5 through the diode D1. As a result, the auxiliary power supply circuit 6 for a normal operation applies the DC voltage generated by the voltage regulator 5 to the PWM control circuit 3 through the diode D2.
In the conventional switching power supply circuit, however, even after the transistor Q1 starts a switching operation and the auxiliary power supply circuit 6 starts to supply power for a normal operation, a current from the rectifier 1 keeps flowing in the resistor R1, resulting in a great loss. In this case, the loss becomes 10 W or more depending on the type and the manner of use of a power supply unit.
This problem becomes conspicuous especially when an operation during a non-load period is to be guaranteed. More specifically, when the auxiliary power supply circuit for the PWM control circuit 3 is simplified most, the power supply unit may not be properly operated without a load. In a general computer or the like, an operation card and the like may be provided as options. In this case, the power supply unit has no load. Assume that such a computer has a function of detecting power abnormality. In this case, if the computer incorporates a power supply unit which is not properly operated during a non-load period, power abnormality is erroneously detected. Therefore, a power supply unit which can be stably operated regardless of the presence/absence of a load is required.
In a power supply unit which provides operation guarantee during a non-load period, since the switching operation of the transistor Q1 becomes intermittent, sufficient power cannot be supplied from the auxiliary winding L3 of the transformer T1. For this reason, most of the driving power required for the PWM control circuit 3 must be supplied through the resistor R1. If the value of the resistor R1 is set in consideration of this situation to supply a current required to drive the PWM control circuit 3, the loss at the resistor R1 is further increased, resulting in a deterioration in the efficiency of the overall power supply unit. Furthermore, in this case, a high-power resistor must be used as the resistor R1, which is large in size, and needs to be spaced apart from other components because it generates heat. Such a resistor inevitably interferes with attempts to reduce the size of the power supply unit.